Add sdl3 renderer demo

[suboyar]

I have ported sdl_renderer to SDL3.

Changes from sdl_renderer to sdl3_renderer:

• Update function names and types for SDL3 API changes
• Remove deprecated flags (e.g. SDL_WINDOW_SHOWN)
• Use the new SDL_PropertiesID system
• Remove legacy Metal cliprect workaround (no longer needed)
• Switch vertex colors from byte values (0-255) to floating point (0-1)

[sleeptightAnsiC]

There is already an advanced PR for SDL3 demo which uses SDL_renderer, see #779
Does this PR have any advantages over the other one? or is it just a duplicate?

[suboyar]

I skimmed through #779, nuklear_sdl3_renderer.h seem quite similar, while their nuklear_sdl3_renderer.c (compared to my main.c) is missing some initialization property and uses SDL's own main callback. While I tried to make a 1:1 implementation of the provided sdl2 demo.

As for why I made this PR, I searched for "SLD3 renderer demo" which didn't yield any results.

[sleeptightAnsiC]

their nuklear_sdl3_renderer.c (compared to my main.c) is missing some initialization property

Is there anything important/critical in those properties? If not, this isn't really a problem since *.c files are more of an example. Having less code in them would be even preferable.

[their nuklear_sdl3_renderer.c] uses SDL's own main callback

It is now preferred to use those callbacks in SDL3, see https://wiki.libsdl.org/SDL3/README-main-functions

I tried to make a 1:1 implementation of the provided sdl2 demo.

Actually, the other PR is missing nk_sdl_handle_grab which I mentioned here #779 (comment) It also does not have Makefile (uses Cmake for some reason) and doesn't follow naming conventions for it's folders/files (compared to other demos).

As for why I made this PR, I searched for "SLD3 renderer demo" which didn't yield any results.

Yea, I also had this issue. I started working with your PR, then realized that #779 exists. IMO, the other PR has really misleading name...

@suboyar Anyway, I'm currently working on #779 since there are more people involved there, but would love to know if the other PR misses anything that your PR solves better.

[sleeptightAnsiC]

Is sdl.ctx.delta_time_seconds being used anywhere? I think that #779 is missing this one. I was also wondering, if we could use delta time for proper double-click (mentioned here)

EDIT: ahh, now I understand, Nuklear needs this for hiding scroll bar!

/tmp/Nuklear/src $ grep -rn 'delta_time_seconds' .
./nuklear_panel.c:493:            window->scrollbar_hiding_timer += ctx->delta_time_seconds;
./nuklear.h:5737:    float delta_time_seconds;